1. Field of the Invention
The present invention relates to a cushioning material for mattresses.
2. Description of the Prior Art
Typically mattresses are required to have the properties of moderate cushioning and permeability. A mattress should be soft, at least on the side of the mattress in contact with the human body. But if the entire mattress is soft, the user will tend to sink into the mattress to too great an extent. This will result in the user feeling uncomfortable in bed, and making the user tired because of the difficulty in turning over in bed. To prevent an unnatural sinking into the mattress of the bed, the cushioning material of the mattress must then have a moderate rigidity.
Conventional mattress cushioning materials that have a moderate rigidity include hard synthetic resin foam board, spring structured materials having many closed springs extending in a number of different directions, and structures having a plurality of crimped fibers, the structures being formed by needling the plurality of crimped fibers and bonding joints of the crimped fibers with adhesive in order to hold the form of the structure, as described in Japanese Patent Early-Disclosure No. 77-152573, and Japanese Patent Publication No. 86-35954. In general, the latter two types of cushioning materials have excellent permeability characteristics. In order to use any of the above-mentioned cushioning materials as a mattress, a soft cushioning material having a relatively smaller spring constant is laminated on both sides of the cushioning material in order to make the mattress softer near the surfaces of the mattress in contact with the human body.
A problem with this type of mattress structure is that recently beds have come into use, particularly the types of beds used in hospitals, which have an undulating bed plate or undersurface. Mattresses used on this type of bed are required to bend according to the undulation of the bed plate. However, the cushioning material structures discussed above, having a moderate rigidity, are difficult to bend due to this rigidity. If this type of mattress having a moderately rigid cushioning material is used on such a bed, partial swelling of the mattress may occur, and deformation of and damage to the mattress is likely to be caused by the bending of the mattress. As a result, the user of such a bed may experience a feeling of discomfort, and the durability of the mattress will gradually be degraded.